The present invention relates generally to the art of bag making. More specifically, it relates to detecting a seal on a bag, and registration to that seal, such as for cutting, perforating etc.
Many different types of plastic bag making machines are known in the art of producing plastic bags for industrial and individual consumers for many different applications (e.g. trash bags). While the present invention has a wide range of applications for the production of such products, the related art will be explained by reference to one particular class of bags i.e., polyethylene trash bags or, garbage bags and wastebasket liners of the type usually sold in boxes of folded bags or rolls of bags.
Examples of prior art machines can be found in U.S. Pat. No. 4,642,084 (the ""084 patent) U.S. Pat. No. 4,934,993 (the ""993 patent), which show generally a bag machine with a rotary sealing drum and U.S. Pat. No. 6,117,058, which shows a bag machine with a rotary sealing drum and servo-driven nips, devices, etc. U.S. Pat. Nos. 4,642,084, 4,934,993 and 6,117,058 are assigned to the owner of this invention and hereby incorporated by reference.
The control of the spacial relationship between a repetitive print pattern on the web and the repetitive seals, or the machine is placing across the web is referred to as the xe2x80x9cregistrationxe2x80x9d of the seal to the print on the web. This spacial relationship may also be referred to as the xe2x80x9cphasexe2x80x9d between the repetitive print and seal occurrences on the web.
Similarly, the control of the spatial relationship between the repetitive seals placed across the web and the repetitive perforations the machine is placing across the web is referred to as the xe2x80x9cregistrationxe2x80x9d of the perforation to the seal on the web. This spacial relationship may also be referred to as the xe2x80x9cphasexe2x80x9d between the repetitive perforations and the repetitive seals across the web. The distance between a seal and a perforation is commonly called the xe2x80x9cskirt lengthxe2x80x9d of the finished bag.
When a bag making machine such as that described in the ""993 patent is used to adjust the drum diameter, any device (such as a perforator, knife, die cutter, punching station, or folding station) on the bag making machine that processes the plastic downstream of the drum may become out of proper synchronization with the sealing process occurring in the drum while the drum is changing diameter. A perforator, for example, slightly out of synchronization causes perforation to seal registration (skirt length) to vary. According to the ""993 patent the skirt length may be adjusted manually. However, by the time the error is detected and the manual correction made, a considerable amount of film may be wasted.
U.S. Pat. No. 5,292,299 (the ""299 patent), incorporated by reference, uses a proximity detector and an encoder to determine where each seal will be placed. However, the ""299 patent does not actually sense and determine the location of a seal; the ""299 patent xe2x80x9cfixesxe2x80x9d the distance between the point of application of the seal and the point of perforation at a constant minimum distance instead of detecting it.
One method of detecting a seal location includes burning a hole in the seal, or providing some other registration mark, and is described in U.S. Pat. No. 5,447,486, incorporated by reference. One significant drawback is that a notch or hole in the seal can be difficult to create, weaken the seal, of fail to be in a position to be detected.
Another prior art method located the seal with respect to a print mark, and then located the perforation to the print mark, thus indirectly locating the perforation with respect to the seal. However, the film path length between the location where the seal is created, and where the perforation is created can be relatively lengthy, thus this method is prone to error. Thus, it is desirable to directly place the perforation with respect to the seal.
U.S. Pat. Nos. 5,518,559, 5,488,480, 5,587,032, 5,660,674 and 5,861,078, hereby incorporated by reference, describe various attempts to register devices to a seal or print mark in bag making machines. Generally, they include optical detectors, that may be more effective than other prior art, but may be costly and/or complex, and still unable to detect all seals. U.S. Pat. No. 5,861,078 describes the use of a xe2x80x9cwindowxe2x80x9d to help reduce false positive detections. The general location of the seal is determined by the location of the seal bar (using encoders for example). Then, the seal sensor is activated for a short period of time (i.e, a window of time or film length) before and after the seal passes by the sensor. The exact location of the seal is then determined by the sensor. This improves the accuracy by eliminating false positives, but it does not decrease the likelihood of a seal being missed by the sensor.
Accordingly, a method and apparatus for detecting a seal is desirable. Preferably, it will not require marks, holes, notches, etc. to be made on the film. Further, it will be preferably be located near the a device which needs to be registered to the seal, and will include a window for greater accuracy.
According to a first aspect of the invention an apparatus for detecting a seal on a film includes a force transmitter and a force sensor. The force transmitter transmits a force from the film to the sensor. The force sensor receives the transmitted force and provides a force signal. A controller receives the force signal and provide a seal signal.
According to a second aspect of the invention a bag machine includes a force transmitter, disposed to transmit a force responsive to a seal. A force sensor receives the transmitted force and provides a force signal. One or more processing devices are located upstream of the force transmitter and one or more processing devices are located downstream of the force transmitter. A controller receives the force signal and creates a seal signal.
According to various alternatives the force sensor is an mechanical sensor, an acoustic sensor, a mechanical sensor, a vibration sensor or a piezoelectric sensor.
An anvil is provided on a first side of a film path, and the force transmitter is on a second side of the film path in another embodiment.
The force transmitter is a quill, preferably rigid, disposed near a path of the film, and the quill may be angled downstream, and have a tip with a radius surface in other embodiments. The quill is held against the film path by a spring force in yet another embodiment.
The controller includes a window circuit in another alternative. The controller includes an amplitude comparator and/or a rise-time comparator in another alternative.
One of the downstream devices, such as a knife, is registered to the seal in another embodiment. The downstream devices and the force transmitter are in a common tension zone in yet another embodiment.
According to a third aspect of the invention a method for detecting a seal on a film includes providing a force signal responsive to the seal and detecting the force and providing a seal signal.
According to a fourth aspect of the invention a method of processing a bag includes transporting the film from a first processing device to a seal sensing location. Providing a force signal responsive to the seal at the seal sensing location. Then detecting the force and providing a seal signal, and transporting the film to a second processing device.
Various alternatives include transmitting a force from the film, preferably using a quill.
Other alternatives include detecting an acoustic, mechanical, or vibration signal.
The seal signal is provided in response to comparing an amplitude of the force with a threshold, and/or comparing a rise-time of the force with a threshold, and/or making such comparisons during a window in several embodiments.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.